Wearable smart devices can provide wearers and other users with useful information. For instance, a wearable smart device can be used to track activity of an elderly person, such as to determine if he has left home unexpectedly. Further, wearable smart devices can be worn to track a person's physical activity, such as how far he has travelled or exercised on a given day. One difficulty with wearable smart devices is providing such devices with power. Typically, a wearable smart device is separated from a user's body to charge. However, while the device is separated from the wearer, the device cannot effectively track activity of the wearer.